1. Field of the Invention
The present invention generally relates to a method of manufacturing a gas seal for a discarding sabot projectile and, more particularly, to manufacturing the gas seal from a chemically modified liquid silicone which is susceptible to polymerization upon exposure to ultraviolet (UV) radiation.
2. Description of the Prior Art
Sabot projectiles are used when launching a subcaliber warhead from an oversized gun barrel. These projectiles comprise plural sabot segment joined about the diameter of the warhead. When a projectile is launched from a gun barrel it acquires its initial velocity from the propulsive effect of the gun propellant gas pressure. The gas pressure is applied over the cross-sectional area of the projectile which is defined by the effective projectile diameter. The sabot segments serve to increase the effective projectile diameter, thereby permitting gas pressure in the gun barrel to act on a larger cross-sectional area. The circumference of the sabot contacts the bore of the gun and acts as a sealing surface which prevents the propulsion gases produced during firing from blowing past the warhead. Once the projectile has traversed the gun barrel, the sabot is discarded with each of the sabot segments dropping away from the warhead. The net effect of the sabot is that the projectile achieves a higher velocity and greater range.
Some of the problems with prior art discarding sabot designs have been related to increased propellant gas pressure and acceleration. The gas pressure and accelerating forces generate stress fields with hoop or circumferential tension components. These stresses frequently cause splits between the sabot segments which prematurely vent the propellant gas. Hence, an additional structural seal has been required for those areas exposed to the propellant gas pressure. In U.S. Pat. 3,620,167, Romer et al disclose a subcaliber projectile with a sabot having a gas pressure receiving surface that is covered with a flat sealing element made of elastic material. The sealing disc is made of a synthetic material and is secured to the aft end of the sabot by gluing or injection molding. In U.S. Pat. 4,598,644, Romer et al disclose a sealing foil made out of propellant charge which prevents gas from blowing past the sabot. The foil consists of desensitized propellant charge powder which has a slower burning velocity than the propellant, thereby assuring a seal between the segments of the sabot until the projectile traverses the gun barrel. In U.S. Pat. 4,424,768, Schiff shows a molded rubber base seal which covers the rearmost end of the sabot. In addition, the sabot includes a ring-like inner segmented seal consisting of three interlocking rubber pieces which provide further protection by preventing gas flow between the projectile and the sabot. At present, gas seals for discarding sabot projectiles are molded. Fabricating synthetic gas seals by molding techniques requires long cure cycle times and mold clean up.
In U.S. Pat. 4,187,783, Campoli et al disclose a discarding sabot munition with a self-adhering base seal member positioned in the aft end of the sabot segments. The seal member comprises a self-curing silicone rubber material such as Silgan H-622 which is manufactured by the SWS Silicone Corporation. The seal is made by extruding the self-curing material into a hole passageway system positioned between the joints of each adjacent sabot segment. The seal adheres to the aft face of the sabot assembly and extends into the seal hole passageway system. This seal is molded in place and requires specially constructed sabot segments.
Unlike some of the organic rubbers, particularly the newer thermoplastic rubbers, silicone rubber must be crosslinked or vulcanized in order to have useful properties. The crosslinking process is referred to as "curing". Free radical producing catalysts such as organic peroxides or azo compounds are mixed with the silicone and are used to initiate the cure reaction. High temperature exposure is used to split the peroxide into a pair of free radicals which will start the polymerization. More recently, liquid silicone rubbers which cure to a solid product at room temperature have been introduced into the market. These are categorically called room temperature vulcanizable (RTV) silicone rubbers and they sometimes are found in paste or semi-solid form. Recently, efforts have been made to use gamma and ultraviolet radiation for curing silicone rubber.
There is no teaching in the prior art of a non-molded, formed in place, gas seal for discarding sabot projectiles which is cured by exposure to UV radiation. Some conventional techniques for manufacturing the gas seal have relied upon materials requiring several hours of oven curing. Other conventional techniques have required substantial investment in expensive capital equipment such as molding machines and precision molds. Drawbacks of the prior art include the fabrication of molds, long heating cycle times, i.e., three to five hours, and mold clean up.